This invention relates to a waveguide device for use in optical communication networks. Specifically, the present invention provides a method for dynamic gain equalization using at least two continuously chirped grating regions to provide overlapping attenuation notches
There is a need for efficient, cost-effective means for controlling wavelength transmission in telecommunications devices especially for Wavelength Division Multiplexing (WDM). WDM utilizes an optical signal that comprises a plurality of discrete wavelength channels. It is important that the power of these channels remains uniform as the optical signal propagates, for example, from a transmitter through an optical fiber and other associated components to a receiver. Inadequate power levels at the receiver can decrease the signal to noise ratio of the system and cause communication errors. To compensate for losses in power that are inherent in the propagation path, it is necessary periodically to boost the individual channels by means of amplifiers placed at junction points. There are several way in which this amplification be achieved using electrical or optical methods. A preferred approach is to use optical amplifiers, such as Erbium-Doped Fiber Amplifiers, which exploit some quantum mechanical effect in a material to create gain in the material and consequent optical amplification of the input signal. Due to the non-linear nature of the gain characteristics of such amplifiers, the amplitude/frequency profile of the output signal will not be uniform, but may exhibit spectral ripple or other variations. Other effects, such as the amplification of spontaneous emission, can increase the bit error rate in individual wavelength channels. There are also other factors that may give rise to wavelength dependent losses and gain, such as absorption, scattering and non-linear fiber effects. Ideally, the output signal should not exhibit ripple but should have a flat profile across the spectrum of its operating bandwidth. The process of correcting these inequalities is referred to as spectral equalization (in the case of the amplitude/wavelength profile) or gain equalization (in the case of the gain/wavelength profile).
A dynamic gain equalization (DGE) device is a specific type of wavelength selective filter used in optical communications systems, typically to compensate for the wavelength-dependent gain variations in amplifiers. Ideally, the DGE should be capable of providing any arbitrary attenuation versus wavelength characteristic. Realistic DGEs are limited by a maximum possible range of attenuation, and by finite wavelength resolution.
Ideally, the output signal should not exhibit ripples but should have a flat profile across the spectrum of its operating bandwidth. Two established methods for implementing spectral or gain equalization are by means of individual channel equalization and by Fourier filtering. In the first technique the individual wavelength channels are separated or de-multiplexed and the power of each channel is separately adjusted using for example liquid crystal devices, following which the channels are recombined or multiplexed. However, such techniques suffer from the problems of insertion loss, polarization dependent loss, dispersion and high cost.
Fourier filtering approaches involve splitting the desired filter profile into a number of frequency components by means of Fourier analysis. The individual frequency components are then adjusted independently of the actual number of channels. There are only a limited number of technologies that enable Fourier filtering. These include Mach-Zehnder based thermo-optic devices and acousto-optic tunable Bragg filters. Mach-Zehnder devices suffer from the problems of heat dissipation and speed, while acousto-optic devices require high power RF signals and carry a significant cost and noise overhead. This approach also suffers from the problems inherent in attempting to synthesize arbitrary spectral profiles from a finite set of filters by a process of Fourier analysis. This results in residual ripple and other non-uniformities.
Domash, in U.S. Pat. No. 5,937,115 describes a family of electro-optical components comprised of an optical waveguide fabricated on, or just under, the surface of a waveguide substrate, a layer of polymer dispersed liquid crystal material (PDLC) in which a Bragg grating has been formed, and a cover plate. One or both of the waveguide substrate and cover plate support electrodes, typically transparent indium tin oxide films, for applying an electric field across the PDLC layer. The Bragg grating is formed in the PDLC layer by first filling the space between the waveguide substrate and cover plate with a liquid mixture of photopolymerizable monomers and liquid crystal material. The liquid material is then illuminated with two mutually coherent laser beams, which interfere to form the desired grating structure. During the recording process, the monomers polymerize and the PDLC mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. When an electric field is applied, the natural orientation of the LC droplets is changed causing the average refractive index and the refractive index modulation of the grating to change. Such components, commonly termed Electrically Switchable Bragg Grating (ESBG) devices are useful as wavelength-selective filters and attenuators in fiber optic communications systems.
U.S. Pat. No. 5,942,157 by Sutherland et al. and U.S. Pat. No. 5,751,452 by Tanaka et al. describe monomer and liquid crystal material combinations suitable for fabricating ESBG devices. A recent publication by Butler et al. (xe2x80x9cDiffractive properties of highly birefringent volume gratings: investigationxe2x80x9d, Journal of the Optical Society of America B, Vol. 19 No. 2, February 2002) describes analytical methods for designing ESBG devices and provides references to publications describing the fabrication and application of ESBG devices.
Ashmead (WDM Solutions, January 2001) described a dynamic gain equalization device comprising a series of Electrically Switchable Bragg Grating (ESBG), each with a different peak wavelength, constructed in series along a planar optical circuit with a single waveguide core.
The object of the present invention is to obviate some of the limitations of the prior art by providing a more optically efficient and cost effective way of synthesizing arbitrary spectral profiles that minimize the problems of ripple and other uniformity variations.
The invention provides a method of filtering an optical signal having a first amplitude frequency or gain frequency profile so as to produce a desired second amplitude frequency or gain frequency profile. Specifically, the method comprises subjecting the signal to action by a plurality of diffraction devices each of which is designed to act upon a respective wavelength band of said signal. Each of said plurality of diffraction devices is controlled to remove at least a part of the radiation in its respective band, such that the combined action of said plurality of diffraction devices is to alter the first profile to the second profile. Advantageously, the proportion of radiation removed by each diffraction device is independently variable such that a wide variety of filter characteristics can be achieved.
The unique features of the invention as compared to the prior art are, first, the grating is fabricated with a continuous chirp, or variation in fringe pitch, along the axis of the waveguide; and, second, the electrodes define a plurality of adjacent independently adjustable filter segments along the waveguide. Each filter segment functions over a specific range of wavelengths, where the center-to-center wavelength difference between adjacent segments and the wavelength bandwidth of each segment are defined in large part by the chirp in the grating.
In a first embodiment of the invention the grating is divided into a group of independently adjustable filter segments each segment having a unique bandwidth and center wavelength, covering the desired wavelength band.
In a preferred embodiment of the invention, the grating is divided into two consecutive chirp regions, both of which span the desired wavelength band. The first chirp region is divided into a first group of independently adjustable filter segments. The second chirp region is divided into a second group of independently adjustable filter segments spaced such that the central wavelengths of the second group of filter segments are interleaved between the center wavelengths of the first group of filter segments. The benefit of this configuration is that it provides a set of overlapping filters functions that can be used to create very smooth attenuation versus wavelength profiles.
A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, wherein like index numerals indicate like parts. For purposes of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail.